The present invention relates to the field of seismic data processing, and in particular to a method for migrating seismic data using a massively parallel computer.
The Earth's subsurface can be imaged by a seismic survey, therefore, seismic data acquisition and processing are key components in geophysical exploration. In a seismic survey, elastic acoustic waves are generated by a source at the Earth's surface and the waves are radiated into the Earth's subsurface. For land seismic surveys, the usual source is dynamite or a seismic vibrator, while for a marine seismic survey the source is typically an airgun array.
As the waves radiate downward through the Earth's subsurface, they reflect and propagate upwards towards the surface whenever the subsurface medium changes. The upward reflections are detected by a number of receivers and the reflected data recorded and processed in order to image the subsurface. Interpretation of these acoustic images of the subsurface formation leads to the structural description of the subsurface geological features, such as faults, salt domes, anticlines, or other features indicative of hydrocarbon traps.
While two dimensional ("2D") seismic surveys have been conducted since the 1920's, three dimensional ("3D") seismic surveys have only recently become widely used. 3D surveys more accurately reflect the subsurface positions of the hydrocarbon traps, but are expensive and time consuming to acquire and process. For an offshore 3D data set covering a 20.times.20 km area, it costs about $3M dollars (1991 dollars) to acquire the data with another $1M dollars for data processing to transform the raw data into useable images. Because the cost of such a seismic survey is considerably less than the cost of drilling an offshore oil well, 3D seismic surveys are often worth the investment.
Although 3D marine surveys vary widely in size (1,000 to 100,000 km.sup.2), a typical marine survey might generate in excess of 40,000 data acquisition tapes. Data is accumulated at a staggering rate, about 1.5 million data samples every 10 seconds. A significant amount of time and money is spent in processing this enormous amount of data.
The result of the seismic survey is thus an enormous amount of raw data indicative of reflected signals which are a function of travel time, propagation, and reflection affects. The goal is to present the reflected amplitudes as a function of lateral position and depth.
A typical marine seismic survey goes through three distinct sequential stages--data acquisition, data processing, and data interpretation. Data processing is by far the most time consuming process of the three. The acquisition time for a medium to large 3D marine seismic survey is in the order of two months. Data is acquired by survey vessels traversing an area of the ocean along a series of parallel lines. A vessel may tow a number of sources (usually airgun arrays) and a number of receiver strings called hydrophone streamers (of length up to 5 kilometers). Sources are fired at 5 to 10 second intervals and the reflected seismic waves measured by up to 1000 hydrophone groups in every streamer. The measurements are recorded digitally on magnetic tapes. In addition to seismic data, navigation information is also recorded for accurate positioning of the sources and receivers. The resulting digital data must then be rendered suitable for interpretation purposes by processing the data at an onshore processing center. The processing sequence can be divided into the following five processing steps.
1. Quality Control, filtering and deconvolution. This processing is applied on a trace basis to filter noise, sharpen the recorded response, suppress multiple echoes, and generally improve the signal-to-noise ratio. Most of these signal processing operations can be highly vectorized. PA1 2. Velocity analyses for migration. This processing estimates the velocity of the subsurface formations from the recorded data by modeling the propagation of acoustic waves with estimated velocities and checking for signal coherence in the acquired data. It is similar to migration but is applied to a small section of the data cube. PA1 3. 3D dip moveout correction and stacking. This processing step, generally the most input/output intensive part of the processing, (i) sums together several traces in order to eliminate redundancy and reduce the signal-to-noise ratio, (ii) corrects for time delays that occur when the reflected signal is recorded by successive hydrophones that are located increasingly farther away from the energy source, and (iii) positions and orients the stacked data in accordance with the navigation information. After this processing step, the data is referred to as stacked data. This step normally constitutes on the order of a 100 to 1 reduction in data volume. PA1 4. Migration. This processing step, computationally the most intensive, relocates the position of reflected strata, that are recorded in time, to their correct position in depth. PA1 5. Enhancement and filtering. This processing step is used to enhance the migrated data using digital filtering techniques.
The stacking process (step 3) reduces the amount of data to what is essentially a three dimensional array of numbers (i.e. a data cube) representing amplitudes of reflected seismic waves recorded over a period of time (usually 8 seconds). Such data cubes can be large, for example, a medium size 3D survey may produce cubes as large as 1000.times.1000.times.2000 of floating-point numbers.
The stacked data cube represents a surface recording of acoustic echoes returned from the earth interior and is not usually directly interpretable. The migration (or acoustic imaging process, step 4) is used to convert stacked data into an image or a map which can then be viewed as a true depth map cut out of the survey area.
Thus, migration is one of the most critical and most time consuming components in seismic processing. Generally speaking, migration transforms the seismic data recorded as a function of time into data positioned as a function of depth using preliminary knowledge of the propagation velocities of the subsurface. In particular, migration moves dipping reflectors to their true subsurface position. Migration is typically performed on post stack seismic data to reduce the amount of processing time, but even so takes weeks of conventional supercomputer time for even medium size post stack seismic data cubes.
Most of the migration methods are based on the one way acoustic wave equation (compressional waves considered, shear waves ignored) using the exploding reflector model. In the exploding reflector model, stacked data are assumed to be recordings of a multitude of sources distributed along geological boundaries and exploded simultaneously. The post stack seismic data cube is considered to be recordings of upward traveling waves as they emerge from the Earth. (See generally, J. F. Claerbout, Imaging the Earth's Interior (1985), M. Dobrin & C. Savit, Geophysical Prospecting (1988), R. E. Sheriff, Geophysical Methods (1989), incorporated by reference for background).
3D seismic migration is becoming the norm in seismic processing and there are two types of methods: two pass and one-pass. In two pass migration, the 3D seismic data cube is migrated sequentially first in the x direction and next in the y direction (orthogonal directions). In one-pass migration an entire 3D data cube is downward continued as one. Various methods for 3D seismic migration have been proposed. (See generally. Blacquiere, Debeye, Wapenaar & Berkhout, 3D Table-Driven Migration, 37 Geophysical Prospecting 925 (1989), Hale, 3-D Depth Migration via McClellan Transformations, 56 Geophysics 1778 (1991), incorporated by reference for background)
The major advantage of one-pass 3D seismic migration is that for structurally complicated areas (faults or strata where steep angles dip) image accuracy is significantly improved. The major disadvantage of the 3D one-pass migration is the significant amount of processing time required. For a typical seismic survey using a terabyte of input data on tapes, the typical 3D processing sequence might take about thirty weeks on a large scale supercomputer. Four to five weeks of processing time is devoted to migration alone. Therefore, it would be a significant advance if an accurate, efficient 3D one-pass migration method were devised.